This would, from my (non-lawyer) understanding, be correct. At least as I
understand US law (other nations may be different), it would be impossible
to force them to reveal sources, since you can't do that with someone
else's code. However...
It's entirely possible, even probable given enough time and the hair
brained legal theories some come up with (the GPL is an anti-trust
violation, riiiggghhttt, glad the Judge thought the same and threw out
that case), that someone (say a competitor) claiming copyright interest in
the kernel might decide to try to force the code out of these people based
on it shipping with the kernel for so long. As I said above, according to
my understanding and by US law at least, that wouldn't be possible.
However, should someone get the hair-brained idea to try it, given the
right circumstances (see SCO), it could drag out forever, costing the
hardware company in question all sorts of money it would certainly rather
be spending elsewhere. Not everyone has the resources of an IBM.
Thus, while it shouldn't be a legal danger in the end, it's enough of an
economic risk thru exposure to simple legal threat, even if ultimately
unsuccessful, that I can easily see a company's accountants and lawyers
combining to veto any permission to ship firmware with the kernel.
But separate it into a non-GPL entangled tarball, and the legal threat
gets much less.
All that said, the threat is really the reverse, as explained by others,
that a SCO could ultimately hold the kernel (at least current and past
shipping and supported versions, and thus everyone using them) hostage,
with things as-is, due to the "if you can't ship it and comply, you can't
ship it" wording), not anyone doing any more than tying up a few legal
resources up for awhile trying to get the code out of the firmware folks.
Thus, while it may be a bit of an economic risk for the firmware folks,
it's a SERIOUS risk for those depending on the kernel, all the more reason
to get the separation done as expeditiously as possible. I'm glad
someone's now attacking the issue, as it /is/ a danger, there are SCOs in
the world, and as such, it needed addressed.